The present invention relates to the field of mass storage devices. More particularly, this invention relates to a disk drive which includes a spindle motor with a hydraulic bearing in the spindle.
One of the key components of any computer system is a place to store data. Computer systems have many different places where data can be stored. One common place for storing massive amounts of data in a computer system is on a disk drive. The most basic parts of a disk drive are a disk that is rotated, an actuator that moves a recording/playback transducer to various locations over the disk, and electrical circuitry that is used to write and read data to and from the disk. The disk drive also includes circuitry for encoding data so that it can be successfully retrieved and written to the disk surface. A microprocessor controls most of the operations of the disk drive as well as passing the data back to the requesting computer and taking data from a requesting computer for storing to the disk.
The transducer is typically housed within a small ceramic block. The small ceramic block is passed over the disk in a transducing relationship with the disk. The transducer can be used to read information representing data from the disk or write information representing data to the disk. When the disk is operating, the disk is usually spinning at relatively high revolutions per minute (xe2x80x9cRPMxe2x80x9d). These days common rotational speeds are 7200 RPM. Some rotational speeds are as high as 10,000 RPM. Higher rotational speeds are contemplated for the future. These high rotational speeds place the small ceramic block in high air speeds. The small ceramic block, also referred to as a slider, is usually aerodynamically designed so that it flies over the disk. The best performance of the disk drive results when the ceramic block is flown as closely to the surface of the disk as possible. Today""s small ceramic block or slider is designed to fly on a very thin layer of gas or air. In operation, the distance between the small ceramic block and the disk is very small. Currently xe2x80x9cflyxe2x80x9d heights are about 1 to 2 micro inches. In some disk drives, the ceramic block does not fly on a cushion of air but rather passes along a layer of lubricant on the disk.
Information representative of data is stored on the surface of the memory disk. Disk drive systems read and write information stored on tracks on memory disks. Transducers, in the form of read/write heads, located on both sides of the memory disk, read and write information on the memory disks when the designated transducer is accurately positioned over the designated track on the surface of the memory disk. The transducer is also said to be moved to a target track. As the memory disk spins and the read/write head is accurately positioned above a target track, the read/write head can store data onto a track by writing information representative of data onto the memory disk. Similarly, reading data on a memory disk is accomplished by positioning the read/write head above a target track and reading the stored material on the memory disk. To write on or read from different tracks, the read/write head is moved radially across the tracks to a selected target track. The data is divided or grouped together on the tracks. In some disk drives, the tracks are a multiplicity of concentric circular tracks. In other disk drives, a continuous spiral is one track on one side of a disk drive. Servo feedback information is used to accurately locate the transducer. The actuator assembly is moved to the required position and held very accurately during a read or write operation using the servo information. It should be noted that the tracks on a disk drive are very thin and closely spaced. Currently, track densities are as high as 15,000 tracks per inch. In practical terms, this means that there are as many as 12 tracks across the width of a single human hair. Of course, track densities will increase in the future.
In the past, the spindle or hub was mounted to the stationary portion of the housing of the disk drive using a race and bearing set. In other words, ball bearings within a race were used to provide an adequately smooth connection between the rotating portion (the hub or spindle) and the stationary portion (fixed shaft) of the disk drive. This is adequate in disk drives that rotated at 3600 rpm or 5200 rpm. Rotational speeds of the disks in a disk drive were then driven to the 7200 rpm level. One reason for higher rpm levels is the constant push by manufacturer""s to minimize access times to data stored on the disk. When the disk pack is rotated at a higher rpm, the access times are less.
It is a well-known fact that as the rotational speeds of a mechanical component increases. The life of the components decreases due to increased mechanical wear. At the 7200 rpm level, manufacturers were seeking different ball bearing materials to extend the life beyond the life associated with metal ball bearings.
Manufacturers are also constantly increasing the capacity of disk drives. One method is to increase the track density. Track density is the number of tracks in an inch. Currently, disk drives have track densities as high as 15,000 tracks per inch. Of course, to increase capacity of disk drives, this parameter will have to be increased. The tracks are very closely spaced. When the tracks are so closely spaced, runout becomes a problem. Runout comprises repeatable runout and non-repeatable runout. Non-repeatable runout is non-repeatable radial motion of the tracks or heads and arises because of mechanical vibrations in the actuator, disk, and spindle assembly. One of the significant contributors to non-repeatable runout is due to spindle bearings. The outer race of a bearing set rotates faster than the inner race of a bearing set. If the inner race of the bearing set is deformed slightly during assembly, or due to some other reason, forces are produced as the outer race travels around the inner race. The forces actually give rise to irregular mechanical vibrations. The non-repeatable runout becomes more of a problem as the track density increases. Another way to state this is that the non-repeatable runout becomes more of a problem as the spacing between the tracks gets smaller. In other words, when the spacing between the tracks becomes sufficiently small, even a small amount on non-repeatable runout makes keeping on track difficult.
To lessen non-repeatable runout, the bearing and race set used in the spindle assembly of disk drives has been replaced by a hydraulic bearing. A hydraulic bearing is also referred to as a hydrodynamic bearing. Spindle assemblies that use hydraulic bearings offer several advantages. One of the main advantages is more accurate disk rotation with less vibration. The end result, is quieter operation with less non-repeatable runout.
Hydraulic bearings do have a problem. The bearing surfaces are large and a lubricant is used to coat the bearing surfaces. The viscosity of the lubricant is lower when the lubricant is at the operating temperature of the disk drive. The viscosity is higher when the lubricant is at a temperature less than the operating temperature of the disk drive, such as at startup of the disk drive. At startup, a high applied torque needs to be applied as a result of the high viscosity of the lubricant at the lower temperature. This leads to designing a spindle motor that has a large starting torque since a large starting torque is needed to start the rotation of the spindle. Generally, a motor which has a high start torque is less efficient when running at the operating temperature. It would be advantageous if a method and apparatus could be used to reduce the viscosity of the hydraulic bearing during the startup time of the motor. Then the spindle motor would require less startup torque and would be more efficient when it ran at operating temperature.
A disk drive system includes a base, a disk stack rotatably attached to the base, and an actuator assembly movably attached to the base. The disk stack is rotatably attached to the housing or the base using a hydrodynamic bearing. The disks are actually mounted or attached to a hub. An electric motor is formed inside the hub. Current is passed through the windings of an electric motor to rotate the hub. The windings are located near the bearing surface of the hydrodynamic bearings. Before applying a current which will start the rotation of the hub, a current is passed through the windings or coil of the electric motor to heat the lubricating fluid used as part of the hydrodynamic bearing. By heating the lubricating fluid, the start torque required to begin the rotation of the hub according to the start profile is reduced. The length of time for application of current at a level which heats the bearing surface and the lubricating fluid at the bearing surface may be controlled by a feedback control loop or may merely be a selected time which does not vary from disk drive to disk drive. Various indicators of the viscosity of the lubricant can be used to monitor the viscosity, if a feedback control loop is used. An adaptive program can also be used based on parameters such as temperature, sensing of winding current, and voltage.
Advantageously, such a system allows disk drives to use electric motors which have lower start torques. These motors are typically more efficient in the run mode when the disk pack is spinning during operation.